A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Alignment between successive patterned layers may be achieved and monitored using markers on the patterning device and on the substrate.
Markers may be used in the alignment of a wafer with respect to the wafer stage and ultimately to the image of the mask. Such markers may be referred to as “wafer align” markers.
Another type of marker can be used in the measurement of alignment that has been performed previously, rather than to achieve alignment itself. Such markers may be referred to as “process correction” markers. These markers are exposed together with the circuit pattern. After the exposure and subsequent processing, such as development and etching, the markers are then inspected using an offline inspection apparatus (measurement tool). This inspection apparatus can measure the difference in placement between markers exposed in two different layers. An example sequence using a wafer substrate is: Expose a first marker in a first layer in resist using a lithographic apparatus. Develop the resist and transfer the pattern into the wafer, by etching. Perform further processing of the wafer (for example addition and/or removal of material). Coat the wafer with resist. Expose a second marker using the lithographic apparatus. Develop the resist. Measure the relative placement between the two markers on the offline inspection apparatus. The measurement data is processed and fed back to the lithographic apparatus. Upon exposing new wafers, this data is used to correct the alignment of the exposure (of either layer).
If the markers of either type on the substrate are not applied properly to the substrate, then the accuracy of alignment may be reduced during the alignment process or correction process respectively.